Pneumatic motorcycle tyre

Abstract: A pneumatic motorcycle tyre comprising a tread portion (2) with axial edges (E1,E2), said tread portion (2) being curved so that the maximum cross sectional width of the tyre lies between the tread edges (E1,E2), a pair of axially spaced bead portions (4), a pair of sidewall portions (3) extending between the tread edges and the bead portions (4), a pair of bead cores (5) one disposed in each bead portion (4), a carcass (6) having a main portion (6a) extending between the bead portions (4) and a pair of turned up portions (6b) each turned up around each said bead core (5), a belt (7) disposed radially outside the carcass (6) and inside the tread portion (2), said belt (7) being formed by spirally winding a ribbon-shaped strip (10) of rubber in which a single cord or a plurality of parallel cords are embedded along the longitudinal direction thereof, at a small angle to the tyre equator (C), characterised by a pair of axially spaced reinforcing cord layers (9) disposed radially inside of the belt (7) and outside of the carcass (6), each reinforcing cord layer (9) extending from a point (P1) in the tread portion (2) on each side of the tyre equator (C) towards the sidewall portion (3) over the belt edge, said point (P1) being spaced apart from the tyre equator (C) such that, when a straight line is drawn normal to the tread surface through the point (P1), the distance measured along the tread surface (2) from the tyre equator (C) to the normal line is in the range of from 0.2 to 0.3 times the tread surface width WT measured along the tread surface from one tread edge (E1) to the other tread edge (E2), each reinforcing cord layer (9) comprises a ply of parallel organic fibre cords (14) inclined at an angle of 20 to 70 degrees to the tyre equator, said ply being rubberised with a topping rubber having a 100% modulus of 25 to 55kgf/cm².

Description: The present invention relates to a pneumatic motorcycle tyre. Recently, as the expressway network is widely developed, the demand for high speed running of motorcycles is increasing. The radial tyres for such motorcycles were hitherto designed and manufactured on the same basis as radial tyres for a four-wheel vehicle, but such designs do not provide the cornering performance and straight running performance which are characteristic of a motorcycle. In particular, high speed steering stability has been inferior. A reason is that a conventional belt 1 reinforcing the tread portion was disposed in the same way as in the four-wheel vehicle tyre, inclined cords (b) at 13 to 30 degrees to the tyre equator as shown in Fig.7(A). Such a belt (a) with inclined cords (b) was, as shown in Fig.7(B), formed by cutting a sheet (d) of tyre fabric having cords (b) extending in the longitudinal direction thereof, at the same angle as the inclination angle (alpha) required of the cords (d) of the belt (a), to form parallelopiped pieces (e), and linking them in a series, abutting the edges (f) of the pieces (e). Thus, the resultant belt (a) has abutting parts (f) extending obliquely to the tyre axial direction. As the length of the abutting part does not contact the ground simultaneously during running, handling is unstable, and the straight running performance is impeded. Further, during cornering, handling is also unstable. These problems occur particularly during high speed running. In order to solve part of these problems, a tyre having a belt formed by spirally winding a long narrow ribbon-shaped strip has been proposed for example in EP-A-0453294. In this tyre, shown in Fig.8, one ribbon-shaped strip (g) is wound continuously from one edge to the other edge of the belt (h). Therefore, the belt (h) has a uniform rigidity or a constant reinforcement for both the tread central region (j) and shoulder region (k). On the other hand, as a motorcycle is greatly inclined during cornering unlike a four-wheel vehicle, a larger camber thrust must be generated during cornering. The above-mentioned tyre with its uniformly reinforced tread portion gives improved straight running performance, but the tread portion is liable to collapse under the lateral force applied during cornering, and the cornering performance is deteriorated. This phenomenon is particularly notable when the belt is formed by spirally winding a ribbon-shaped strip. As the ribbon-shaped strip is wound spirally, the cords in the ribbon-shaped strip are wound at nearly 0 degrees to the tyre equator. As a result, the resistance of the carcass and belt to lateral force is poor. In order to increase the resistance to lateral force, a reinforcing cord layer (m) made of a sheet of conventional tyre fabric is disposed over the entire width of the belt (g) as shown in Fig.9. Thus the rigidity of the tread shoulder region is improved, and the camber thrust occurs and becomes a large value early as the motorcycle makes a turn, so that the cornering performance can be improved. However, the ground contact and steering stability during straight running are deteriorated since the tread central region (j) is increased in bending rigidity. EP-A-0219311 discloses a radial motorcycle tyre according to the preamble of claim 1 having a breaker in the central portion of the tread and a reinforcing layer disposed between the carcass and breaker in each of the tread shoulders. The tyre combines the manoeuvrability of a cross-ply tyre with the durability, high speed performance and wear resistance of a radial tyre. It is therefore, an object of the present invention to provide a pneumatic motorcycle tyre which is improved in both cornering stability and straight running stability. According to one aspect of the present invention, a pneumatic motorcycle tyre comprises a tread portion with axial edges, said tread portion being curved so that the maximum cross sectional width of the tyre lies between the tread edges, a pair of axially spaced bead portions, a pair of sidewall portions extending between the tread edges and the bead portions, a pair of bead cores one disposed in each bead portion, a carcass having a main portion extending between the bead portions and a pair of turned up portions each turned up around each said bead core, a belt disposed radially outside the carcass and inside the tread portion, a pair of axially spaced reinforcing cord layers disposed radially inside of the belt and outside of the carcass, each reinforcing cord layer extending from a point in the tread portion on each side of the tyre equator towards the sidewall portion over the belt edge, said point being spaced apart from the tyre equator such that, when a straight line is drawn normal to the tread surface through the point, the distance measured along the tread surface from the tyre equator to the normal line is in the range of from 0.2 to 0.3 times the tread surface width WT measured along the tread surface from one tread edge to the other tread edge, characterised by said belt being formed by spirally winding a ribbon-shaped strip of rubber in which a single cord or a plurality of parallel cords are embedded along the longitudinal direction thereof, at a small angle to the tyre equator, each reinforcing cord layer comprising a ply of parallel organic fibre cords inclined at an angle of 20 to 70 degrees to the tyre equator, said ply being rubberised with a topping rubber having a 100% modulus of 25 to 55 kgf/cm 2 and each reinforcing cord layer covering the edge of each turned up portion of the carcass. Accordingly, as the belt is formed by spirally winding a single cord or a plurality of cords at a small inclination angle to the tyre equator, the belt is continuous in the circumferential direction in contradistinction to the conventional cut ply belt shown in Fig.7(B). Therefore, the running stability and ride comfort are improved. Since the reinforcing cord layer extends from the middle point P1 between the tyre equator and tread edge into the sidewall portion, the tread shoulder region is reinforced more than the tread central part. Therefore, collapse or distortion of the tread portion caused during cornering by the bank angle of the motorcycle can be prevented. Further, the low or medium speed cornering is made stable since the rigidity of the tread shoulder region is increased. Also, the high speed cornering becomes stable since the belt cord is continuous in the circumferential direction. Further, since the reinforcing cord layers are disposed radially inside the belt, the straight running is stable and the straight running performance is not lowered, even though the cords are not parallel to the tyre equator. Furthermore, since the reinforcing cord layer is formed independently from the belt and carcass, the tread rigidity can be adjusted easily, as compared with such a tread portion reinforced with a multiply belt or an extension of the carcass turned up portion. If the distance of the point P1 is less than 0.2 times the tread width WT, the width of the overlap of the reinforcing cord layer with the belt is excessively increased, and the ride comfort and ground contact performance during straight running are deteriorated. Further, the tyre weight, fuel consumption, running noise increase and production cost increase. If the distance is more than 0.3 times WT, the running stability becomes lowered during slow cornering at a small bank angle since the reinforcing cord layer is located away from the ground contacting region. If the 100% modulus of the topping rubber for the reinforcing cords is less than 25 kgf/cm 2 , the lateral rigidity from the tread shoulder region to the sidewall portion becomes insufficient, and the camber thrust also becomes insufficient for the camber angle, and the time lag for the generation of camber thrust becomes large, thereby resulting in unsteadiness of the machine, and the high speed steering performance is deteriorated. To the contrary, if the 100% modulus is more than 55 kgf/cm 2 , the ride comfort during straight running is greatly deteriorated. In the present invention, thus, the cornering performance and straight running performance are improved. Embodiments of the present invention will now be described in detail in conjunction with the drawings. Fig.1 is a cross sectional view showing an embodiment of the present invention; Fig.2 is a cross sectional view showing another embodiment of the present invention; Fig.3 is a cross sectional view showing still another embodiment of the present invention; Fig.4 is a developed plan view of the belt and the reinforcing cord layers showing an example of the cord arrangement; Fig.5 is an enlarged schematic perspective view of an example of the ribbon-shaped strip; Fig.6 is a schematic sectional view explaining an example of the spiral winding of the ribbon-shaped strip; Fig.7(A) and (B) are diagrams showing conventional belts; and Figs.8 and 9 are cross sectional views each showing a prior art tyre. In Figs.1-3, a pneumatic motorcycle tyre 1 comprises a tread portion 2 with a tread surface 2A and tread edges E1 and E2, a pair of axially spaced bead portions 4 each with a bead core 5 therein, and a pair of sidewall portions 3 extending between the tread edges and respective bead portions. Further, the tyre 1 comprises a carcass 6, a belt 7 and a pair of bead apexes 8. In the tyre meridian section, the tread surface 2A between the tread edges E1 and E2 is provided with a curved profile generally formed as a circular arc, and the maximum tyre section width lies between the tread edges E1 and E2. The carcass 6 comprises at least one ply, in Figs.1, 2 and 3 only one ply, having a main portion 6a extending between the bead portions 4 through the tread portion 2 and sidewall portions 3, and a pair of turned up portions 6b each turned up around each of the bead cores 5 from the axially inside to outside of the tyre. The or each carcass ply is made of parallel cords arranged radially at an angle in the range of from 70 to 90 degrees with respect to the tyre equator C to provide a radial or semiradial ply construction. For the carcass cords, organic fibre cords, e.g. nylon, rayon, polyester, aromatic polyamide or the like are used. The radially outer end of the carcass turned up portion 6b is positioned radially outwardly from the bead portion 4 to near the belt edge in Figs.1 and 3 or a position about 50 % of the tread edge height HE in Fig.2. Each of the bead apexes 8 is disposed in its bead portion 4 between the carcass main portion 6a and the turned up portion 6b and radially outside the bead core 5. The bead apex 8 is made of rubber having a Shore A hardness in a range of 40 to 95 degrees and has a triangular shape tapering radially outwards from the bead core. The upper end of the bead apex is positioned near to the tread edge. The belt 7 is disposed radially outside the carcass 6 and inside the tread portion 2. The belt 7 consists of one ply of a single cord or a plurality of parallel cords wound spirally around the carcass 6 at a small angle of not more than 5 degrees with respect to the tyre equator C. For such belt cords 14, organic fibre cords, e.g. nylon, polyester, aromatic polyamide or the like or steel cords can be used. Preferably, the width WB of the belt 7 is in the range of 0.85 to 0.95 times the tread width WT. Here, the belt width WB is measured along the outer surface of the belt, and the tread width WT is measured between the edges F1 and F2 along the tread surface. If less than 0.85 times, the rigidity of the tread edge portion is decreased, and the steering stability is impaired during cornering with the machine largely inclining. If more than 0.95 times, the rubber thickness from the belt edge to the sidewall surface becomes small, and the upper sidewall portion is liable to crack decreasing tyre durability. The above-mentioned belt ply is formed by spirally winding a long continuous narrow-width ribbon-shaped strip 10. The ribbon-shaped strip 10 is, as shown in Fig.5, formed by one belt cord or a plurality of parallel belt cords, in this example two cords 14, embedded in topping rubber 15. Such a ribbon-shaped strip 10 is wound continuously from one belt edge F1 near one tread edge E1 to the other belt edge F2 near the other tread edge E2, across the tyre equator C. In the winding operation of the ribbon-shaped strip 10, as shown in Fig.6, the confronting side edges 10a and 10a of the strip 10 can be overlapped to prevent loosening. In all the embodiments shown in Figs.1, 2 and 3, the belt 7 is formed by a single ply 7a, but the belt may be of two or more plies. In other words, the ribbon-shaped strip 10 may be wound double or more. According to the present invention, a pair of axially spaced reinforcing cord layers 9 are disposed radially inside the belt 7 and outside the carcass 6. Each reinforcing cord layer 9 is made of one ply of rubberised parallel organic fibre cords 16, e.g. nylon, rayon, aromatic polyamide or the like, laid at an angle of 20 to 70 degrees to the tyre equator C. With respect to the tyre equator, the cords of one layer 9 are inclined reversely to the cords of the other layer 9. For the topping rubber for the reinforcing cords, a rubber composition having a 100% modulus of 25 to 55 kgf/cm 2 is used. Each of the reinforcing cord layers 9 extends along the outer surface of the carcass 6, axially outwardly from a point P1 between the tyre equator and tread edge, over the belt edge F1 and F2, and then radially inwardly towards the sidewall portion 3. The point P1 is spaced apart from the tyre equator C as follows. When a straight line N is drawn normal to the tread surface 2A through the point P1, the distance measured along the tread surface 2A from the tyre equator C to the normal line N (point O) is in the range of from 0.2 to 0.3 times the above-mentioned tread width WT. Preferably, the distance is 0.25 times the tread width WT. When a motorcycle is inclined by giving a camber angle in order to change the running direction, if a large time lag occurs until the generation of camber thrust, the driver feels it is difficult to run in the intended course, and increases the camber angle to generate further chamber thrust. However, after a certain time lag, excessive camber thrust then occurs which causes an over-steering effect. It is therefore, important to shorten the time lag. Thus, such a time lag occurs unsteady feelings to the driver in case of motorcycles. The time lag can be shortened by increasing the lateral rigidity of the tyre, especially in the region from the tread shoulder region 12 to the sidewall portion 3. For straight running, however, it is necessary to decrease the rigidity of the tread centre region 11 since the central region contacts the ground during straight running, and it is important to improve the ground contact for manoeuvrability and wear resistance. According to the present invention, the reinforcing cord layers 9 range from the tread shoulder region 12 to the sidewall region, and the angle of the reinforcing cords is crosswise to that of the carcass cords. Therefore, the lateral rigidity of the tyre when a camber angle is given thereto, is increased to shorten the time lag, and the steering stability is improved. Further, the reinforcements in the central region 11 are only the belt cords 14 laid substantially parallel to the circumferential direction and carcass cords laid generally in the axial direction. Accordingly, this region is low in bending rigidity. As a result, vibrations during running due to road undulations and slip between the road surface and tyre are effectively decreased, and the tread wear life is prolonged. Further, running noise and harshness are effectively alleviated. The other end P2 of each reinforcing cord layer 9 is positioned in the range R1 of from 0.1 to 1.0 times the tread edge height HE which is the radial distance from the baseline L to the tread edge E1 and E2. In Fig.1, the end P2 is positioned at a height slightly lower than the tread edge height HE so that the layers 9 reinforce only the tread shoulder region 12. In Fig.2, the end P2 is positioned at a height H2 slightly higher than 10 % of the tread edge height HE so that the layers 9 reinforce both the tread shoulder region 12 and sidewall portion 3. By covering the edge of the carcass turned up portion 6b within the reinforcing cord layer 9, carcass ply edge separation can be prevented to improve the durability. In Figs.1 and 2, in the sidewall portion 3 and bead portion 4, the reinforcing cord layer 9 is disposed axially outside the carcass turned up portion 6b and not turned up around the bead core 6. In Fig.3, however, the reinforcing cord layer 9A is extended to the bead base and turned up around the bead core 5, to further extend along the axially inside of the carcass main portion 6a. In this case, also the end P3 thereof is positioned at a height H3 in the same range R2 of from 0.1 to 1.0 times the tread edge height HE. In this example, the end P3 is positioned slightly lower than the tread edge height HE. Accordingly, the sidewall portions 3 are effectively reinforced, and the high speed cornering performance and the engaging force with the rim are effectively improved. Test tyres having a tyre size of 190/50 R17 were prepared and tested for the following performances. The specifications of the test tyres and test results are shown in Table 1. 1) High speed straight running stability and High speed cornering stability Test tyres were installed on a motorcycle, and the motorcycle was run on a test road at a speed of 260 km/h on a straight course and at 220 km/h on a 400 meter radius curved course. The performance was evaluated by the feeling of the test driver, and expressed as an index based on the prior art tyre being 100. The greater the index, the better the performance. 2) Running stability during slow and Middle speed cornering Running on a curved road at a speed of less than 100 km/h, the test tyres were tested the same as in 1). The result was expressed as an index based on the prior art tyre being 100. The greater the index, the better the performance. 3) Durability Each test tyre, which was inflated to 2.5 kgf/cm 2 and loaded with 130% of the maximum load specified in the Japanese Industrial Standard, was run at a speed of 50 km/h, using a drum testing machine. The result was expressed as an index based on the tyre which was successful in 15000 kilometre running without damage as index 100. 4) Vibration ride comfort The ride comfort was evaluated by the feeling of the test driver through the test 1), and expressed as an index based on the prior art tyre being 100. The greater the index, the better the performance. 5) Tyre weight The tyre weight was measured, and expressed as an index based on the prior art tyre being 100. The greater the index, the heavier the tyre weight. Through the tests, it was confirmed that the example tyres were excellent in both high speed and slow speed cornering performances as compared with the prior art tyre and comparative example tyres. Further, the high speed straight running performance was not inferior as compared with the comparative example tyres. As explained above, in the pneumatic motorcycle tyre of the present invention, the belt is formed by spirally winding a ribbon-shaped strip, and a pair of axially spaced reinforcing cord layers are provided inward of the belt to reinforce the tread shoulder region. Therefore, the cornering performance during both high speed and slow speed running can be improved without lowering the high speed straight running performance.

Claims: A pneumatic motorcycle tyre comprising a tread portion (2) with axial edges (E1,E2), said tread portion (2) being curved so that the maximum cross sectional width of the tyre lies between the tread edges (E1,E2), a pair of axially spaced bead portions (4), a pair of sidewall portions (3) extending between the tread edges and the bead portions (4), a pair of bead cores (5) one disposed in each bead portion (4), a carcass (6) having a main portion (6a) extending between the bead portions (4) and a pair of turned up portions (6b) each turned up around each said bead core (5), a belt (7) disposed radially outside the carcass (6) and inside the tread portion (2), a pair of axially spaced reinforcing cord layers (9) disposed radially inside of the belt (7) and outside of the carcass (6), each reinforcing cord layer (9) extending from a point (P1) in the tread portion (2) on each side of the tyre equator (C) towards the sidewall portion (3) over the belt edge, said point (P1) being spaced apart from the tyre equator (C) such that, when a straight line is drawn normal to the tread surface through the point (P1), the distance measured along the tread surface (2) from the tyre equator (C) to the normal line is in the range of from 0.2 to 0.3 times the tread surface width WT measured along the tread surface from one tread edge (E1) to the other tread edge (E2), characterised by said belt (7) being formed by spirally winding a ribbon-shaped strip (10) of rubber in which a single cord or a plurality of parallel cords are embedded along the longitudinal direction thereof, at a small angle to the tyre equator (C), each reinforcing cord layer (9) comprising a ply of parallel organic fibre cords (14) inclined at an angle of 20 to 70 degrees to the tyre equator, said ply being rubberised with a topping rubber having a 100% modulus of 25 to 55 kgf/cm 2 and each reinforcing cord layer (9) covering the edge of each turned up portion of the carcass (6). A tyre according to claim 1, characterised in that, with respect to the tyre equator (C), the cords (14) of one of the reinforcing cord layer (9) are inclined reversely to the cords (14) of the other reinforcing cord layer (9). A tyre according to claim 1 or 2, characterised in that each reinforcing cord layer has a radially inner edge (P2) not turned up around the bead core and terminated at a height in the range of from 0.1 to 1.0 times the height of the tread edge, each measured from the baseline. A tyre according to claim 1 or 2, characterised in that each reinforcing cord layer has a radially inner edge (P2) turned up around the bead core from the axially outside to inside of the tyre and terminated at a height in the range of from 0.1 to 1.0 times the height of the tread edge, each measured from the baseline. A tyre according to any of claims 1 to 4, characterised in that the ribbon-shaped strip (10) is wound continuously from one belt edge F1 to the other belt edge F2 across the tyre equator C. A tyre according to any of claims 1 to 5, characterised in that the belt (7) consists of a single ply (7a). A tyre according to any of claims 1 to 6, characterised in that the width (WB) of the belt (7) is in the range of 0.85 to 0.95 times the tread width (WT). A tyre according to any of claims 1 to 7, characterised in that each reinforcing layer (9) consists of a single ply.

Inventors: SUZUKI SHIGEHIKO, SUZUKI, SHIGEHIKO
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1994
Main classification: B60C9
IPC codes: B60C15, B60C9